In recent years, internal combustion engines have been expected to work with improved thermal efficiency because of increasing environmental awareness regarding, for example, reduction of fossil fuel use, reduction of carbon dioxide emissions, and prevention of global warming. Heat engines such as gas turbines or jet engines are known to maximize thermal efficiency by operating with a high-temperature Carnot cycle at higher temperatures. As temperature rises at the turbine inlet, the importance of development and modification of materials used for gas turbine hot parts (i.e., combustors, turbine rotor blades, and stator blades) increases. In order to cope with such temperature elevation, Ni-based heat-resistant alloys exhibiting excellent high-temperature strength are employed as materials, and many Ni superalloys are employed at present. Ni-based alloys are classified as standard casting alloys comprising equiaxial crystals, unidirectionally solidified alloys comprising columnar crystals, and single-crystal alloys comprising a single crystal. In order to strengthen Ni-based alloys, it is necessary to add large quantities of solid-solution strengthening elements, such as W, Mo, Ta, and Co, and to allow large quantities of γ′Ni3 (Al, Ti) phases (i.e., strengthening phases) to precipitate with the addition of Al and Ti.
Meanwhile, there is a tendency to use low-quality fuels containing large quantities of impurities causing corrosion as land-based gas turbine fuels due to elevated fuel prices. Accordingly, development of materials exerting high-temperature strength and corrosion resistivity is also necessary. Large quantities of Cr that form protective coating are preferably added in order to prepare such materials. Examples of alloys in which corrosion resistivity is regarded as important include standard casting alloys represented by Patent Document 1 or Patent Document 2.
As alloy element content increases, however, the tissue stability of materials deteriorates, and hard and fragile harmful phases (e.g., sigma phases) are disadvantageously precipitated after prolonged use.
In the past, specifically, it has been difficult to develop alloy materials excellent in creep strength at high temperatures, corrosion resistivity, and oxidation resistivity.